The present invention relates to an improved signal transmission fuse, such as shock tube, of the type used for transmitting a detonation signal and, more particularly, to an improved tape-containing structure of such fuse, and to a method of making the same.
Signal transmission fuses of the type commonly referred to as shock tube are well-known in the art. U.S. Pat. No. 3,590,739, issued Jul. 6, 1971 to Per-Anders Persson, discloses a hollow elongated plastic tube having a pulverulent reactive substance, which may be constituted by a highly brisant explosive such as PETN, RDX, TNT or HMX, adhered by various means to the interior wall of the shock tube.
U.S. Pat. No. 4,328,753, issued May 11, 1982 to L. Kristensen et al, discloses a low energy fuse in the form of a plastic tube comprised of concentric tubular plies of material on the inner surface of which is disposed a pulverulent reactive material. One of the problems which Kristensen et al seeks to redress is the art-recognized problem of migration of the reactive material powder from the inner surface of the tube to form a loose powder in the tube. Kristensen et al does this by making the inner or sub-tube of a polymeric material, such as an ionomeric plastic of the type sold under the trademark SURLYN by E. I. Du Pont de Nemours and Company (xe2x80x9cDu Pontxe2x80x9d), to which the pulverulent reactive material will cling. The patentee states that the reactive material will be dislodged substantially only by the shock wave generated by reaction of the explosive powder. While ionomers such as SURLYN plastics provide good adhesion of such reactive material, such ionomers are susceptible to degradation by ultraviolet radiation, have unacceptably high water vapor and oil permeabilities, and are insufficiently tough for field use. Kristensen et al offers as a solution surmounting the sub-tube with an outer tube made of a less permeable and mechanically tougher material such as a polyamide, polypropylene, polybutylene or other such polymer better able than the sub-tube to withstand the environment and the stresses of deploying the flise at a work site. The reactive material is a powdered mixture of an explosive and aluminum powder and Kristensen et al discloses that the adhesive nature of the sub-tube enables adherence of about 7 grams of explosive powder per square meter of surface of the inner surface of the tube. Test data are presented that show dislodgment by mechanical forces of about 3 to 61 percent by weight of the amount of reactive material initially present on the inner surface of the tube, depending on the particular type of SURLYN material used for the sub-tube.
Ionomers of the SURLYN type are also advantageous for deposition of the pulverulent reactive material thereon because they can be reliably extruded at a relatively low temperature of about 185xc2x0 C. (As described below, the reactive material is deposited on the inner surface of the tube at the tube extrusion head.) A reactive material powder containing a thermally stable explosive such as HMX, which has a degradation temperature of about 275xc2x0 C., can safely be deposited directly on a plastic which is at or near its extrusion temperature of about 185xc2x0 C. However, the extrusion temperature of SURLYN plastics is too high to permit use of less expensive explosives such as PETN, which has a melting point of only about 141xc2x0 C., or even RDX, the 204xc2x0 C. melting point of which is less than about 20xc2x0 C. higher than the lowest SURLYN plastic extrusion temperature. The thermally less sensitive explosives, such as HMX, are not only more expensive, but are less sensitive than explosives such as PETN and RDX, therefore reducing the reliability of initiation of the signal transmission fuse.
As is well known in the art, the pulverulent reactive material is introduced into the SURLYN or other ionomer tube at the point at which the tube is being extruded, the reactive material powder normally being fed by gravity concentrically within the parison being pulled from the extrusion head. It has been found that extremely fine particles of such reactive material are difficult to uniformly and reliably apply by gravity flow. This problem is overcome by using a somewhat larger particle size of the reactive material, but the larger particle size results in aggravating the problem of migration of the powder from the tube surface because the larger particles, being heavier, adhere less well to the tube inner surface.
The use of a larger particle size of the reactive material also tends to reduce the sensitivity of the reactive material to initiation, thereby requiring depositing somewhat heavier loadings of the reactive material powder which, in turn, further aggravates the powder migration problem.
Powder migration is a problem because, in products where lengths of the signal transmission fuse are connected to devices such as detonators, migrating powder can collect atop the explosive or pyrotechnic contained within the detonator and shield the explosive or pyrotechnic from the signal generated in the shock tube, thereby resulting in a misfire. Further, deployment of a shock tube in the field results in bends and kinks in the shock tube, and a collection of migrated powder can block the shock tube at such bends or kinks, thereby interrupting transmission of the signal and also resulting in misfire. Of course, if powder migration is so severe as to leave sections of the fuse with insufficient powder adhered thereto to sustain the reaction, a misfire will occur.
Despite the problem of powder migration, the art has persisted in using a loose pulverulent reactive material in signal transmission fuses such as shock tube, deflagrating tube and the like, because it is believed that the reactive material, which is believed to be retained on the ionomer only by Van der Waals forces or the like, must be dislodged at the point of reaction so that it can react, in a manner analogous to a dust explosion, to sustain the reaction and transmit it through the entire length of the tube.
Russian Patent 2,005,984 of Pechenev et al, entitled xe2x80x9cInitiating Waveguidexe2x80x9d, discloses a signal transmission fuse (which is referred to as xe2x80x9can initiating waveguidexe2x80x9d in the translation of the Russian Patent). The Patent discloses applying the reactive mixture (xe2x80x9cexplosivexe2x80x9d) on a film at a core loading of 5 to 40 g/m2, the film being enclosed within a surrounding sheath or tube xe2x80x9cwith a gap of 0.5 to 7 mmxe2x80x9d. The Russian Patent thus provides a film or tape to which an explosive powder is applied and which is then encased within a surrounding tube to provide the finished xe2x80x9cinitiating wave-guidexe2x80x9d or signal transmission fuse.
U.S. Pat. No. 4,290,366, issued Sep. 22, 1981 to F. B. Janoski, discloses a signal transmission tube within the bore of which is disposed a self-oxidizing material which extends substantially throughout the length of the tube. The self-oxidizing material may comprise a monofilament or a multifilament of fine, hair-like strands of material that loosely fills the flexible tubing and which may carry explosive modifying materials to alter the density and/or detonation rate of the self-oxidizing material.
The prior art also uses, as a fuse, cotton strings or cords coated with black powder and contained within a hollow plastic tube. The black powder is mixed with a binder to adhere it to the strings or cords.
The present invention provides a fuse structure and method of making a fuse which overcomes the foregoing problems.
Generally, in accordance with the present invention, there is provided a signal transmission fuse in which a support tape has a reactive material containing a binder coated onto the tape. The reactive material, which may comprise known explosive/fuel mixtures or deflagrating compositions, or a mixture thereof, may be applied to the tape in the form of a reactive paint comprising the pulverulent reactive material, a binder and, optionally, a solvent. The coated tape is then encased within a tube, which may be a plastic (synthetic organic polymeric) tube, which is extruded or otherwise applied over the tape, so that the support tape separates the coating of reactive material from the, for example, hot, freshly extruded, plastic tube. The reactive material is thereby protected from contact with the hot, freshly applied outer tube and this gives more flexibility in selecting both the reactive material and the plastic because the degradation temperatures (defined below) of the components of the reactive material, such as an organic explosive, and the temperature at which the plastic tube is applied, are no longer constraining factors. The utilization of a binder retains the reactive material on the tape during manufacture and, in the finished product, prevents migration of the reactive material through the signal transmission fuse and enables the use of greatly increased core loadings of the reactive material. The increased core loadings may be made high enough so that, when an explosive/fuel mixture is employed as the reactive material, the signal transmission fuse is desirably ruptured upon use.
Specifically, in accordance with the present invention, there is provided a signal transmission fuse comprising the following components. A tube has a longitudinal axis and a tube wall which defines a tube outer surface and a tube inner surface, the tube inner surface defining a bore extending through the tube. A support tape has a first side and an opposite second side and a reactive coating on the first side of the support tape. The reactive coating comprises a reactive material (for example, a pulverulent mixture of an organic explosive and an oxidizable fuel, and/or a pulverulent deflagrating mixture) and a binder. The weight of binder in the reactive coating is less than the weight of the reactive material in the reactive coating, but sufficient to cause the reactive coating to adhere to the first side of the support tape more strongly than it would if the binder were absent. The support tape is disposed within, and extends along the bore of the tube, with the second side of the support tape facing the tube inner surface, and leaves an open portion of the bore extending through the tube adjacent to the reactive coating.
In one aspect of the present invention, the support tape is configured as a channel so that, in cross section, the first side of the support tape is of concave configuration and the second side of the support tape is of convex configuration.
In another aspect of the present invention, substantially all of the second side of the support tape is disposed in contact with the tube inner surface.
Other aspects of the present invention provide for particular reactive materials, as described below, to be applied with a suitable binder as a coating on the support tape.
Still other aspects of the present invention provide for the support tape to comprise a laminate tape in which the first side is comprised of a material, e.g., polyethylene terephthalate, to which the reactive coating is adherent and the second side is comprised of a material, e.g., polyethylene, which is adherent to the tube inner surface.
Yet another aspect of the present invention provides for the tube, or at least the inner surface thereof, to be comprised of a synthetic polymeric material and at least the second side of the support tape to be comprised of a synthetic polymeric material which is bondable to the tube inner surface. For example, in one embodiment of the invention, at least the inner surface of the tube and at least the second surface of the support tape are each comprised of mutually bondable, or the same, or chemically identical, synthetic organic polymers.
A method aspect of the present invention provides for making a signal transmission fuse by the following steps. There is provided a support tape having a first side and an opposite, second side. A reactive coating comprising a binder and a pulverulent reactive material (which may be one or both of an explosive/fuel mixture or a deflagrating composition) is applied to the first side of the support tape to provide a coated support tape having a reactive coating on the first side thereof. The coated support tape is then formed into a channel configuration to provide it with, in cross section, a convex exterior defined by the second side and a concave interior defined by the first side of the support tape. A tube, for example, a synthetic polymeric tube, is applied over the formed support tape, the tube having a tube inner surface which faces the second side of the formed support tape and defines a bore extending through the tube and within which bore the formed support tape is contained. The concave interior of the folded support tape defines an open portion of the bore which extends longitudinally through the tube adjacent to the reactive coating.
In another aspect of the present invention, the support tape may be supplied at a temperature (including ambient temperature) which is below the degradation temperature (as defined below) of a reactive material comprising an explosive and a binder. For example, the support tape may be supplied at a temperature which is at least 20xc2x0 to 30xc2x0 C. below the degradation temperature of the reactive material, e.g., the tape may be supplied at ambient temperature.
Another method aspect of the present invention provides for the reactive coating to be applied as a reactive paint comprising the pulverulent binder, the pulverulent reactive material and a solvent, and the solvent is evaporated to provide the reactive coating.
In one aspect of the present invention the method includes placing substantially all of the second side of the support tape in contact with the inner surface of the tube.
Another aspect of the present invention includes applying the reactive coating to the support tape as a reactive paint comprising the pulverulent binder, the pulverulent reactive material and a solvent, and evaporating the solvent to provide the reactive coating.
In specific aspects of the method of the invention, the specific materials mentioned above are used to produce the signal transmission fuse.
Other aspects of the invention will be apparent from the following description of specific embodiments thereof.
Unless specifically otherwise stated, as used herein and in the claims, the following terms have the indicated meaning.
The term xe2x80x9c% by weightxe2x80x9d or the like used with respect to a particular component of the reactive coating or otherwise, means the weight of the component as a percent of the total weight of the reactive coating or other material, including the particular component, on a dry (solvent-free) basis.
The term xe2x80x9corganic explosivexe2x80x9d means a nitro-organic compound explosive such as PYX, HNS, RDX, PETN, etc. (These abbreviations, and others, are defined below.)
The terms xe2x80x9cchannelxe2x80x9d, or xe2x80x9cchannel configurationxe2x80x9d, or xe2x80x9cchannel-like configurationxe2x80x9d used to describe the support tape, means that the support tape is formed or folded to have a convex exterior and concave interior, and the terms include channels which are U-shaped in cross section (xe2x80x9copen channelxe2x80x9d) and O-shaped in cross section (xe2x80x9ctunnelxe2x80x9d).
The term xe2x80x9cdegradation temperaturexe2x80x9d, e.g., as applied to a material such as a reactive material, reactive coating, reactive paint or components thereof, means that temperature at or above which desired properties of the material will be adversely affected, e.g., the material or a component thereof may melt or otherwise be adversely affected.